(1) Field of the Invention
The present invention relates to optoelectronic data communications systems, and in particular to such systems where digital data is transmitted via TTL compatible circuits to drive a light emitting diode (LED) at high frequencies, and in which the LED is coupled to an optical fiber to transmit optical pulses corresponding to digital input signals to an optoelectronic receiver at the other end of the fiber.
(2) Description of the Prior Art
Interference-free optoelectronic data transmission systems as a means for transmitting information have been considered for approximately the past 25 years. During the research and development phases of practical systems, a great deal of effort has been directed toward economic production of rugged fibers, low voltage photodetectors, long life light sources and reliable connectors. However, as these problems have been solved, more attention has been directed toward a practical and efficient means of interfacing these optical systems with the electronic systems that constitute the rest of the total information transmitting system.
A particularly troublesome problem relating to such interfaces relates to the aspect of driving a suitable light source such as an LED at high frequencies in response to a conventionally encoded input digital signal. One approach to providing an LED driver circuit is disclosed in U.S. Pat. No. 3,968,399 (Jarrett), in which emitter coupled logic (ECL) gates are utilized. While such an approach enables high frequency energization of LED's, it requires both positive and negative power supplies and is not directly compatible with TTL logic signals most often encountered in data communication systems. While some circuits are known for driving LED's using TTL gates, it is not known to provide a simple and inexpensive means enabling 30 MHz bandwidth. See "Optoelectronics and interface Electronics", P. W. Casper, SPIE Vol. 63 (1975) Guided Optical Communications, pp. 19-27, at page 21. Circuits such as there set forth produce significant current overshoots, i.e., up to 25%, particularly in the trailing edge of LED energization pulses, with attendant "ringing", such that the LED may not be properly de-energized between successive pulses, resulting in erroneous signal transmissions.